The present invention relates to the depression of precipitation of sparingly soluble salts, such as calcium carbonate, on contact-surfaces and in the bulk phase of industrial and domestic water systems.
It is known that in domestic water systems and in many aqueous industrial processes, sparingly soluble salts, and particularly salts having inverted solubility, such as calcium carbonate (CaCO3), are readily precipitated on contact-surfaces of piping and equipment. Scale formation is a particular problem in heat exchangers and cooling towers, where scale precipitation reduces heat-transfer efficiency, raises energy cost, and decreases the flow-rate and capacity of the system.
There are many techniques used for preventing scale. One widely used technique consists of adding trace concentrations of chemicals capable of decreasing the scale precipitation phenomenon.
Most of the presently used chemicals are polymeric materials like polyphosphates, phosphonates and polyacrylates, usually in the presence of minute amounts of metal ion impurities, such as Fe+2, Mg+2, Cu+2 and Zn+2. Many of these additives promote formation of non-adhering aragonite crystals, rather than the usual, adherent calcite morphology. It has been found that zinc ions are particularly effective in decreasing the adherence of calcium carbonate on contact-surfaces.
The following references exemplify these prior-art techniques. U.S. Pat. No. 4,529,572 to Baumberger et al. teaches the use of a zinc complex of an 80:20 acrylic acid-ethylacrylate copolymer for scale prevention in re-circulating water systems. Japanese Patent 01299700 to Ito describes a method using water-soluble organic phosphonates, water-soluble zinc or molybdate salts; and water-soluble copolymers of methacrylic acid and N-substituted methacrylamide. WO 2002049960 to Mueller discloses tripolyphosphates including cations such as, NH4, Li, Na, K, Mg, Sr, Mn, Zn, and Ti, for use as scale inhibitors.
It should be noted that these techniques require chemical dosage pumping devices to provide the desired quantities of scale-inhibiting compounds. Moreover, these techniques often require complicated, online, continuous feedback controlling systems.
In a markedly different approach, the introduction of zinc ions has been achieved using electrochemical reduction-oxidation reactions. Thus, U.S. Pat. No. 5,837,134 to Heskett, teaches the use of particles of a copper-zinc alloy for preventing scale on contact surfaces in water. The zinc and copper in the alloy form a galvanic cell, thereby liberating zinc ions in the water.
One of the disadvantages of the art taught by Heskett is that the surface area of the copper-zinc alloy particles changes as a function of time. Consequently, the concentration of zinc ions in the water is not constant and is largely uncontrolled. Perhaps more significantly, the concentration of zinc ions can increase to undesirable levels when the flow rate of water is low or when the flow is closed for long durations.
It should also be mentioned that excessive concentrations of zinc ions liberated into aqueous systems may precipitate ZnCO3, or Zn(OH)2, and form colloidal masses that require a filtration system.
It is known that water quality influences the precipitation of sparingly soluble salts, in general, and of CaCO3, in particular, on the contact-surfaces of water systems. The rate of formation of scale layers significantly increases as supersaturation of these salts exceeds a certain critical level. One effective way of expressing the supersaturation level of CaCO3 is the Langelier Saturation Index (LSI). The LSI is an equilibrium model derived from the theoretical concept of saturation that provides an indicator of the degree of saturation of water with respect to calcium carbonate. The LSI approximates the base 10 logarithm of the calcite saturation level. The Langelier saturation level approaches the concept of saturation using pH as a main variable. The LSI can be interpreted as the pH change required to bring water to saturation with respect to calcium carbonate.
Thus, water with a LSI of 1.0 is one pH unit above saturation, hence, reducing the pH by 1 unit will bring the water to saturation. This occurs because the portion of total alkalinity present as CO32− decreases as the pH decreases, according to the equilibria describing the dissociation of carbonic acid:H2CO3⇄HCO3−+H+HCO3−⇄CO32−+H+When the LSI is negative, there is no potential to scale and the water will dissolve CaCO3; when the LSI is positive, scale can form and CaCO3 precipitation may occur; and when LSI is close to zero, the water is close to the saturation point, such that a slight change in water quality or changes in temperature, or evaporation could result in precipitation.
It must be noted that the LSI is purely an equilibrium index and deals only with the thermodynamic driving force for calcium carbonate scale precipitation and growth, in terms of pH as a master variable. The LSI provides no indication of how much scale or calcium carbonate will actually precipitate or dissolve as equilibrium is approached.
In order to calculate the LSI, it is necessary to know the alkalinity (milligrams per liter as CaCO3), calcium hardness (milligrams per liter Ca2+ as CaCO3), total dissolved solids (TDS) (milligrams per liter TDS), actual pH, and the temperature of the water (° C.). Over the usual pH range of natural waters, the LSI is given by the following expression:LSI=pH−pHs wherein pH is the measured pH of the water, and pHs is the pH at saturation in calcite or calcium carbonate and is defined as:pHs=(9.3+A+B)−(C+D)wherein:A=(Log10[TDS]−1)/10,B=−13.12*Log10(° C.+273)+34.55,C=Log10[Ca2+ as CaCO3]−0.4, andD=Log10[alkalinity as CaCO3].
It would be advantageous to have a water treatment method and device that is adaptive to the LSI, or that is largely insensitive to changes therein.
It must be emphasized that while U.S. Pat. No. 5,837,134 to Heskett explicitly refers to scale reduction, the technique taught by Heskett achieves the reduction of scale on contact surfaces by deliberate precipitation of the scale, as Heskett himself articulates:                . . . it is believed that the zinc component of the brass enters into the crystal growth of the scale weakening it and causing it to disintegrate and fail so that the scale is flushed out with the hot water.        (column 4 lines 56-59)        
U.S. Pat. No. 4,789,448 to Woodhouse discloses a device for preventing downstream scale formation by an inline configuration that includes a zinc anode and a copper cathode externally connected to an electrical circuit. This device releases charged ions into the water so as to form heterogeneous nucleation sites for crystallization of suspended impurities in the water. Consequently, precipitation on the contact-surfaces of vessels and pipes is diminished. U.S. Pat. No. 4,789,448 to Woodhouse explicitly teaches that it is desirable to deliberately precipitate suspended solids in the water by means of the sacrificial zinc anode. The zinc ions and the negatively charged salt particles that are released into the water by the device, form                sites in the water for formation of crystals of scale forming impurities which remain in suspension in the water thereby reducing deposition of scale on the surfaces of vessels, pipes, or the like into or through which the water subsequently flows. (column 1, lines 50-54)These suspended salts and impurities may block the downstream system and sometimes may require filtration. In any event, the particulate matter remains in the bulk phase, which is particularly disadvantageous in many applications.        
There is therefore a recognized need for, and it would be highly advantageous to have, a device for, and a method of, depressing the precipitation of sparingly soluble salts, in general, and of calcium carbonate, in particular, while inhibiting the formation of suspended solids. It would be of particular advantage for such a device and method to be simple and robust, inexpensive, non-toxic, and facile to control and operate.